RNA Interference against the Three Citrus Tristeza Virus Genes Encoding Silencing Suppressors Confers Complete Resistance to the Virus in Transgenic Mexican Lime Plants

Abstract

Citrus tristeza virus (CTV), the causal agent of the most devastating viral disease of citrus, has evolved three silencing suppressor proteins acting at intra-(p23 and p20) and inter-cellular level (p20 and p25) to overcome host antiviral defense. RNA interference (RNAi), an approach based on using dsRNA to trigger RNA silencing, has been widely used for generating transgenic plants resistant against viruses. Considering the important role of p23, p20 and p25 in CTV pathogenesis, we obtained transgenic Mexican lime plants with an intron-hairpin vector carrying full untranslatable versions of genes p25, p20, p23 and the 3'-UTR from the CTV strain T36, to attempt silencing their expression in CTV-infected cells. Complete resistance to viral infection was observed in three transgenic lines, with all their propagations remaining symptomless and virus-free after graft-inoculation with CTV-T36, either in the non-transgenic rootstock or directly in the transgenic scion. Accumulation of transgene-derived siRNAs was necessary but not sufficient for CTV resistance. When immune transformants were challenged with a dissimilar CTV strain the resistance was partially broken, stressing the importance of sequence identity in the underlying RNAi mechanism. This is the first evidence that it is possible to achieve full resistance to CTV in a highly sensitive citrus host by targeting simultaneously its three viral silencing suppressors through RNAi.